This invention relates generally to optically pumped lasers, and more particularly to a diffraction grating coupled optically pumped submillimeter laser system.
Submillimeter laser technology finds applicability in many new areas. Perhaps the most immediate requirement for submillimeter systems is in plasma diagnostics. Submillimeter lasers can be used in the CW mode in interferometric arrangement or pulsed in high power Thompson scattering experiments. In either case the submillimeter frequencies are uniquely capable of making measurements at fusion densities and temperatures. For example, a typical angle of Thompson scattering for 500-.mu. radiation from a dense plasma at a T.sub.i (ion temperature) of 2keV would be about 30.degree.. This is in contrast to a CO.sub.2 laser which requires a scattering angle of less than 1.degree.. This small-angle scattering suffers from severe stray light problems and from a lack of flexibility necessary to avoid the effects of plasma-density fluctuations.
The wavelengths of submillimeter lasers also are particularly appropriate for high-resolution, all-weather aircraft landing systems. Narrow beamwidths achievable with practical-size antennas imply superior angular resolution at these frequencies. The wide video bandwidths available make narrow pulse widths and excellent radial resolution available. Submillimeter lasers also offer very attractive characteristics for short-range surveillance radars.
While near-infrared and visible lasers offer better performance under clear-weather conditions, in dense fog, where the radar is most needed, the shorter wavelengths are severely scattered. The droplet size distribution of typical fogs and clouds peaks in the range of 1.mu.m to 10.mu.m. Since particle scattering efficiency decreases rapidly for wavelengths greater than the particle size, the scattering of submillimeter radiation is extremely small compared to the visible. In thick clouds, typical estimates for the submillimeter scattering losses are about 20dB/km versus 400dB/km at 0.63.mu.m. The greater vapor absorption of the submillimeter radiation is well compensated by the low scattering losses under these adverse conditions.
Another potentially important application of these short wavelengths, is in modeling radar systems. Measurements made within hours in the laboratory with model ships and tanks can save time and money in construction of new systems. The size of scale models required at these wavelengths is well suited to machine tolerances and critical measurements.
Secure military communications (ship-ship, ground to air or satellite, air-air) can use the high absorption of H.sub.2 O in the atmosphere at particular frequencies and altitudes effectively. It is possible to choose wavelengths where horizontal air to air absorption is less than 0.5dB/km at an altitude of 10 km but where the air to ground path is essentially opaque. It is possible also to operate at wavelengths where the vertical path attenuation from ground to air is acceptable while the horizontal ground-level absorption is extremely high. By correct selection of optical-pumped lasers it is possible to design communications which, in addition to being secure from interception, are virtually impossible to detect.
In the area of basic research submillimeter lasers have already provided a useful source for the magnetospectroscopy of solids. By studying the absorption of radiation in high-purity semiconductors as a function of magnetic fields, impurity concentrations of less than 10.sup.11 cm.sup.-3 have been identified. Cyclotron-resonance observations of the free electrons produce data on the mobility and effective masers, orders of magnitude more accurate than obtained via indirect measurements. The lasers themselves have yielded new information about the excited states of the lasing molecular gases. The constants determined in this way are extremely valuable for the field of molecular spectroscopy. In the field of astrophysics, where investigation of interstellar molecular sources at submillimeter wavelengths is just beginning, future detectors also can be expected to use these optically-pumped lasers (in conjunction with wideband Schottky-diode mixers) as local oscillators in sensitive heterodyne receivers.
Already, flow discharge molecular lasers such as H.sub.2 O, D.sub.2 O, HCN, DCN, SO.sub.2 and H.sub.2 S have significantly changed the nature of submillimeter techniques. But, in the variety of molecules it can excite the flow-discharge technique has gone as far as it can go in emission lines and power available. Optical pumping, on the other hand, makes possible the desirable features of near infrared lasers available for far-infrared or submillimeter applications.
Optically pumped lasers, however, are generally cumbersome in design and extremely inefficient in operation. Since the application of submillimeter lasers are greatly increasing it becomes essential to provide optically pumped submillimeter lasers which are both compact in design and highly efficient in operation.